People who sit for hours on long trips should know about a condition called deep vein thrombosis. A thrombosis is a blood clot, a situation where some blood thickens and blocks the flow. Clots develop deep inside the legs when blood cannot move easily back to the heart. Blood clots can kill if they move to the heart and lungs.

Doctors say some people have an increased risk of clots. These include people who have had clots in the past, as well as pregnant women and those who take birth control pills. People who are overweight and those with heart disease or cancer also may have a greater risk. Others include people being treated with estrogen, and those who have had a recent operation.

Experts say travelers should drink plenty of water -- not liquids that contain alcohol or caffeine. Another thing to do is to increase the blood flow to the legs. This could mean wearing support stockings or taking an aspirin a few hours before the trip. Also, people should not sit for a long time with their knees pressed back against their seat. Walk around every hour or so. Or at least make sure to move the feet and legs.

Doctors say anyone who has pain, swelling or red skin on a leg during or after a long trip may have a blood clot. Signs that a clot may have already reached the lungs include chest pain, shortness of breath, coughing and a fast heart rate. In many cases, the condition can be treated with drugs that thin the blood and prevent clots from moving through the body.